1. IntroductionPast accident history suggests that, despite the fact that severe scenarios in large industrial plants are quite rare, they can lead to dramatic consequences for people’s health, the environment, business, and asset integrity [1,2]. In recent years, the robustness of safety methodologies has considerably increased to guarantee that the technological development is accompanied by adequate safety considerations.In this sense, particular attention is paid to risk-relevant plants (chemical, oil and gas, nuclear, etc.) in which dangerous substances and equipment are involved, as demonstrated by the strict regulations introduced to enhance their safety demonstration methodologies [3,4].

This work focuses on the oil and gas (O&G) offshore activities, and in this framework, the reference regulation is the EU Offshore Safety Directive 2013/30/EU, which defines the quantitative risk assessment (QRA) as the most suitable tool for safety assessment.

In particular, where natural gas extraction is concerned, one of the major safety issues is related to the presence of hydrogen sulfide (H2S) traces, as it is a flammable and toxic gas with the potential to cause major injuries and death; in this case, stricter safety constraints are imposed on the extraction process of CH4-H2S mixtures (sour gas) [5].According to [6], the O&G industry is one of the most significant anthropogenic sources of H2S, and nowadays, several O&G extraction plants perform activities that involve sour gas, as reported by several works, including the works mentioned in the following section. In the North Sea, natural gas can contain up to 2 vol% of H2S [7]. The Kazakh gas fields of Kashagan and Tengiz present contents of H2S around 19 vol% and 16 vol%, respectively [8,9]. In the Sichuan Basin, the percentage of H2S in natural gas can vary between 10 vol% and 17 vol% [10], while in the Eastern Venezuela Basin, the concentration can be ~5 vol% [11].Due to its corrosive nature, H2S can cause several types of damage to industrial components, increasing their failure rate, and hence their release frequency [12]. An accidental release of sour gas can lead to several major accidents, such as explosions, fires or toxic dispersions and the consequences can be dramatic for the people and the environment, as observed during the Lodgepole blowout accident in Alberta in 1982 [13]. In fact, it can cause severe eye and respiratory irritation, and, at high concentrations, it can cause immediate human deaths [14]. It is also highly dangerous for vegetation, as high concentrations can inhibit root growth and compromise plant growth [15]. If it is dispersed in water, it can affect the marine fauna, as exposure to fishes causes hyperpnea, followed by final respiratory arrest [16]. For all these reasons, the evaluation of the consequences of an accidental sour gas release must be carefully addressed during the risk assessment of those plants.The state-of-practice for the accident simulation for QRA purposes entails the use of simple tools based on empirical correlations, such as turbulent free-jet models [17], gas dispersion models [18] and jet fire models [19,20], which guarantee a fast response with no need for deep theoretical knowledge. On the other hand, these methods neglect geometry (i.e., the flow–object interaction is not modeled), and lead to highly conservative results with a consequent waste of resources when over-protection measures are implemented. This last consideration is true especially in the case of congested plants, such as offshore platforms, where a limited space is available, and in nuclear plants, where a major part of the equipment is inside a containment building to avoid external leakage of radioactive materials.Complex methods, such as computational fluid dynamics (CFD), might be employed to guarantee the needed accuracy. Several authors have already used CFD approaches to reproduce accidents, such as the sour gas well blowout accident that occurred at Kaixian (China) in 2003, which caused 243 deaths [21,22].Furthermore, CFD is widely used to simulate accidental gas dispersions, e.g., accidental H2 dispersions in automotive scenarios [23,24], CO2 releases from high-pressure pipelines [25,26] and high-pressure natural gas releases from pipelines [27]. In particular, a novel assessment tool based on CFD is proposed in [28,29,30] to estimate the extent of high-pressure methane jets impinging on differently shaped objects.

Despite its extensive use for single scenarios, CFD can be hardly integrated in a QRA, due to its expensive computational cost. Its response time is too long to permit the estimation of the consequences of hundreds of accidental scenarios during the plant design phase.

For this reason, the SEADOG laboratory at Politecnico di Torino proposed a novel approach called the source box accident model (SBAM) [31,32,33,34], which is based on ANSYS Fluent and used for the simulation of high-pressure gas releases in congested environments, guaranteeing a sustainable accuracy and computational cost compromise. The approach involves CFD and is based on the splitting of the accidental release phenomenon into the following two main physical steps: the supersonic release near the release hole and the far-field dispersion.In this work, SBAM is employed to simulate a high-pressure sour gas release in a platform under several wind conditions. A sensitivity analysis of the wind speed is presented, since this represents a key parameter for the assessment of consequences related to hazardous substances’ releases [35].In the literature, there are already several works that focus on wind sensitivity analysis of gas dispersions for QRA purposes. In [36,37], CFD analyses of the influence of wind on flammable gas clouds for hazardous area classification and on the H2S dispersion consequences are respectively proposed. In [38], gas dispersion wind is analyzed through the empirical-based software PHAST and in [39], a CFD-based empirical model for hazardous area classification is proposed. It must be remarked that none of these studies consider real-life complex geometrical arrangements because of the high computational cost and the associated complexity. To close this gap, this work proposes an alternative approach, employing SBAM, which allows us to deal with high-pressure releases, congested environments and high gradients in the main parameters (such as concentrations and speed).

The first objective of the present work is to describe an efficient way to perform a set of simulations with different wind conditions via CFD, without the need for high computational efforts. In fact, when dealing with external flows, CFD could require a prohibitive computing power, as large computational domains are needed to correctly reproduce the flow-field around the object of interest (e.g., a plant). Secondly, the wind intensity impact on the resulting damage areas and risk-related figure of merits is analyzed.

In the next section, the case study is introduced, as well as the flammability and toxicity limits related to the involved gas mixture. Section 2 is devoted to the methodology, and it is divided into the following two parts: the first part is related to the wind field simulation, while the second part is related to the dispersion. In Section 3, the results related to the gas dispersion under several wind conditions are discussed, and some conclusions and useful insights are proposed in Section 4. Case StudyFigure 1 shows a platform geometry, in which the main components are represented by simple geometric shapes and secondary equipment is neglected.The domain ceiling is represented as transparent for visual purposes. For the sake of simulation, it is a plated ceiling. The cardinal points south (S), east (E), north (N) and west (W) are indicated to univocally identify the lateral faces of the platform. The domain dimensions are Lx = 30 m, Lz = 20 m and H = 5 m. The release position is represented by the yellow bullet in Figure 1, and its coordinates are x = 3 m, y = 2.5 m and z = 10 m, while the release direction is x→. The wind direction is schematically represented in the upper left corner of Figure 1. The other relevant parameters are as follows:

Release pressure and temperature: prel = 50 bar and Trel = 300 K;

Release hole diameter: drel = 3 cm;

Released gas mixture mole composition: 99 mol% CH4, 1 mol% H2S;

Wind velocities: Uref = 2-4-6-8-10 m/s;

Ambient temperature: Ta = 300 K.

The release pressure and hole diameter (when a circular hole is assumed) are chosen in agreement with [40], which suggests the most frequent values for these parameters basing on the review of loss of containment accidents in the O&G field. Trel is defined by assuming that the leakage involves a piece of equipment, such as a pipeline or a tank, in thermal equilibrium with the external environment. The mixture composition that is chosen is realistic, considering that a natural gas well with a much higher H2S concentration is unlikely to be exploited due to its toxicity level. Different values of Uref are chosen to assess their impact on flammability and toxicity quantitative consequences, in terms of volumes and masses. The following flammability and toxicity limits for the involved substances are introduced to determine the zones of installation where a flammable or toxic concentration can be reached in case of an accident:

LFL: lower flammability limit;

UFL: upper flammability limit;

IDLH: immediately dangerous to life and health concentration;

LC50: lethal dose at which 50% of the population is killed in a given time following exposure.

The reference values for each species taken from [41,42,43] are presented in Table 1. Since both gases are flammable, the LFL and UFL of the released mixture can be evaluated using the Le Chatelier’s rule, as was the case in [44], considering that the mole concentration of CH4 (CCH4) is 99% and the H2S mole concentration (CH2S) is 1%. where i indicates the i-th species. Considering an analogous formula for the UFL and the values in Table 1, the following values are obtained for the mixture:

LFLmix = 4.994%;

UFLmix = 16.104%.

These flammability limits values are considered for the representation of the flammable areas and the calculation of the flammable quantities in the discussion of results in Section 3. 4. Conclusions

In this paper, a method to reproduce a wind field to be used in several gas dispersion simulations is presented and is used to perform a wind sensitivity analysis of a high-pressure sour gas release on an offshore platform.

The wind field is obtained by simulating the air flow around the platform, considering a sufficiently large external box. The latter is dimensioned, assuring that a fully developed wind velocity profile impacts the platform. The results show that the external box sides must be at a distance from the platform boundaries that is equal to 3 times the platform height. This methodology allows computational cost savings for the following reasons:

A new criterion for the dimensioning of the wind simulation domain is defined; the volume is reduced by ~80% with respect to the state-of-the-art methodologies.

The gas dispersion can be simulated in a smaller domain (in this case, in the platform) and not in a larger external box.

Once the wind field is available, the results can be used to simulate several dispersion simulations.

Moreover, the wind fields related to the wind speeds equal to 2-4-6-8-10 m/s are calculated and used in the dispersion simulations, realized through SBAM, to assess their influence on the flammable and toxic consequences.

The case studies are characterized by a release of sour gas (99% CH4-1% H2S) at 50 bar from a circular hole with a diameter of 3 cm. The results show that, despite the very low amount of H2S released, the threat zones related to toxicity are relevant. In particular, the irreversible injuries areas related to toxicity are larger than the flammability areas, because of the very low IDLH of the H2S (100 ppm).

The dangerous zones and toxicity and flammability areas are strongly influenced by the wind intensity. In fact, as the wind increases, the dilution is enhanced, and the dangerous volumes decrease. Nonetheless, after a certain value of wind speed, the dangerous volume variation is less accentuated, and saturation effect appears, except for the IDLH volume.

Wind intensity is a crucial parameter for the assessment of the consequences of the accidental release of hazardous gases, as it largely affects the damage areas, especially in cases of low wind conditions.

Indeed, before taking decisions and defining additional preventive/protection measures, a careful analysis must be performed, considering both the obtained results and other interesting parameters, e.g., the type of equipment involved in the dangerous volumes, the distribution of the workers (people) inside the platform, etc.

The methodology shows that it is possible to save computational effort when performing these kinds of analyses, which are likely to be included in a QRA. In fact, threat areas and volumes obtained through these kinds of simulations can serve as input parameters for event tree analysis (ETA) in order to obtain a risk estimation [55]. Other useful insights can be gained by the results obtained from a contingency planning point of view, since safer areas on the platform could be identified, as well as preferable escape routes on the plant perimeter.